This invention relates to methodology for the rapid non-destructive screening of samples of polymer composites and coatings. In particular, it relates to a method for the rapid determination of the light-induced degradation of polymeric materials and coatings. Additionally, the methodology may be applied to combinatorial libraries of such composites and coatings.
A significant factor in the outdoor weathering lifetime of a polymeric material is the ultraviolet radiation (UV) dosage received by a given sample. Although outdoor weathering data is considered most useful for the direct evaluation of a material""s UV stability, the fundamental limitation of such techniques is the need for multi-year exposure times. Thus, other weathering approaches are also used as a more attractive alternative to more rapidly evaluate and qualify materials for particular applications in an effort to judge acceptable performance of the polymer or coating as well as screening additives to optimize efficacy.
Outdoor weathering studies are typically performed at test sites in Florida or Arizona for durations of about one to ten years. Reductions in testing times can be achieved by using accelerated laboratory methods such as ASTM G-26 xenon arc lamp conditions. Even running at high irradiance, it takes about 1100 h of sample exposure to accumulate 2700 kJ/m2 at 340 nm which is approximately equivalent to one year of Florida exposure. Thus, laboratory accelerated weathering generally provides acceleration factors of 3 to 10 for most materials.
Degradation of organic materials such as polymers and coatings by ultraviolet light is often manifested by a loss in gloss and/or a yellowing in color, especially in white pigmented materials. A widely used analytical technique for determination of UV degradation involves measurements of the Yellowness Index (YI) according to ASTM procedure D-1925. Accordingly, a lower change in YI (xcex94YI) suggests higher photostability of a sample, and thus better weatherability. However, for reliable quantification, it is desirable to reach a xcex94YI value of 3 or more, which often corresponds to at least several hundred kJ/m2 of irradiation at 340 nm. Moreover, reliable measurements of YI cannot be performed after short exposure times, i.e., several hours, because of the relatively low sensitivity of YI determination at low UV exposures.
An additional 10-1000 fold acceleration of the weatherability studies is needed for analysis of multiple samples such as those created as combinatorial libraries. Unfortunately, currently available analytical methods lack the sensitivity to detect sample properties such as YI and others after only several hours of UV exposure and correlate them with the ultimate weatherability of the sample. Moreover, the color shift in black or darkly colored samples is often relatively insensitive to UV exposure. Color measurement in other samples containing pigments and dyes can be complicated by color shifts in the colorants themselves. Thus, often only gloss retention measurements are done on dark and black samples, which usually require even larger UV exposure doses for reliable determinations of a given sample""s UV stability, since often gloss loss is apparent only after the surface is heavily degraded.
Thus, it would be desirable to develop a method for reliable quantification of weatherability of different types of samples, including dark and black materials, at exposure times of only hours. In addition, it would be desirable to be able to calculate the useful lifetime of a given material.
This invention provides a method for the rapid determination of the light-induced degradation of polymeric materials and coatings. In this method, samples are exposed to UV light and fluorescence emission spectral data are collected and fluorescence properties such as wavelength intensity, lifetime and polarization are monitored for changes, which in turn are an indication of the weatherability of a given sample.